1. Field of the Invention
The present invention relates to a thermosensitive printer, and more particularly to a thermosensitive color printer where a full-color image is recorded on a thermosensitive color recording material in a three-color frame sequential fashion while the thermosensitive color recording material is being moved back and forth relative to a thermal head.
2. Background Arts
The thermosensitive color recording material used in the thermosensitive color printer has thermosensitive coloring layers for cyan, magenta and yellow formed atop another on one side of a base material, and a transparent protection layer is formed on an uppermost coloring layer. Hereinafter, the thermosensitive color recording material will be called simply the color recording material. As the color recording material moves relative to the thermal head that is pressed on the top side of the color recording material, the thermal head heats the color recording material such that the coloring layers develop the three colors sequentially from the upper layer to the lower layer. To stop the previously colored layer from coloring again by heat energy applied for the next coloring layer, the upper two coloring layers, e.g. yellow and magenta coloring layers, are designed to be fixed by ultraviolet or near-ultraviolet rays of specific wavelengths respectively before starting recording on the next layer.
The thermal head is provided with a heating element array that consists of a large number of heating elements aligned along a transverse direction to the moving direction of the color recording material. The heating elements are located on a ridge or summit of a semi-cylindrical protrusion that extends in the transverse direction, so that the heating elements are brought into tight contact with the color recording material, and thus the heat energies are efficiently transmitted to the color recording material. The semi-cylindrical protrusion is provided as a portion of a glaze layer that is formed on an aluminum substrate.
Meanwhile, as disclosed for example in Japanese Laid-open Patent Application No. 2000-71495, a teaching to displace the center of a platen roller from a center of the heating element by an appropriate offset amount in an appropriate offset direction has been know in the art. This is for the purpose of optimizing the contacting condition of the color recording material with the heating element array, and thus obtaining adequate graininess of the recorded image. As described in this prior art, graininess of the recorded image becomes adequate when the color recording paper is cooled moderately after being heated by the heating elements.
Because the thermosensitive color printer disclosed in this prior art is of a type that records the image only while the color recording paper is being conveyed in a predetermined direction, the position of the platen roller relative to the heating element array is fixed at an optimum offset position for any colors. However, there is a type of thermosensitive color printer where the thermal head records one color frame as the color recording material moves in one direction, and a next color frame as the color recording material moves reversely. In this type of printer, adequate graininess cannot be obtained by displacing the center of the platen roller constant amount and direction from the center of the heating element array.
In view of the foregoing, an object of the present invention is to provide a thermosensitive printer that provide optimum contacting conditions of the thermosensitive recording material with the heating element array during recording.
Another object of the present invention is to provide a thermosensitive color printer that provide optimum contacting conditions of the thermosensitive color recording material with the heating element array during recording any color frames, even where the recording is executed alternately in opposite paper conveying directions.
According to an aspect of the present invention, in a thermosensitive printer that records an image on a thermosensitive recording material by heating the thermosensitive recording material through an array of plurality of heating elements of a thermal head as the thermosensitive recording material is conveyed perpendicularly to the array of the heating elements, the thermosensitive printer comprises a platen roller placed in opposition to the array of the heating elements, for supporting the thermosensitive recording material from its back side while the heating elements are pressed onto an obverse side of the thermosensitive recording material; and an offset adjusting device for adjusting offset amount and offset direction of a rotary center of the platen roller from a center of the heating element in a conveying direction of the thermosensitive recording material.
According to a preferred embodiment, the center of the platen roller is displaced by a predetermined amount upstream from the center of the heating element in the conveying direction of thermosensitive recording material.
According to another aspect of the present invention, in a thermosensitive color printer that records a full-color image on a thermosensitive color recording material having thermosensitive coloring layers for yellow, magenta and cyan, by heating the thermosensitive color recording material through an array of plurality of heating elements of a thermal head, wherein the thermosensitive color recording material is conveyed alternately in opposite directions perpendicular to the array of the heating elements, to record the full-color image in a three-color frame sequential fashion, the thermosensitive printer comprises a platen roller placed in opposition to the array of the heating elements, for supporting the thermosensitive color recording material from its back side while the heating elements are pressed onto an obverse side of the thermosensitive color recording material; and an offset adjusting device for adjusting offset amount and offset direction of a rotary center of the platen roller from a center of the heating element along the conveying directions of the thermosensitive color recording material.
The offset adjusting device preferably displaces the center of the platen roller upstream from the center of the heating element in either conveying direction of the thermosensitive recording material.
According to another preferred embodiment, the offset amount of the center of the platen roller from the center of the heating element is predetermined for each color.